Fuel cell systems are known in various forms. For example there are, amongst others, the following types of fuel cells:
PEM fuel cells (Proton Exchange Membrane)
DMFC fuel cells (Direct Methanol Fuel Cell)
SOFC fuel cells (Solid Oxide Fuel Cell)
MCFC fuel cells (Molten Carbide Fuel Cell)
PAFC fuel cells (Phosphoric Acid Fuel Cell) and
AFC fuel cells (Alkaline Fuel Cell).
Of these fuel cell types at least PEM fuel cells, which are fed with hydrogen or with a hydrogen-rich synthesized gas, require a certain relative humidity at the hydrogen side of the fuel cells in order to be able to operate. For this reason it has previously been customary to provide a water container at the anode side of the fuel cells which delivers water to the humidifying device for the humidifying of the hydrogen gas.
Furthermore, similar humidifying devices are provided at the cathode side of the fuel cells and ensure that the oxygen (normally in the form of air) supplied to the fuel cells at the cathode side of the fuel cells is appropriately humidified. The humidifying of the gases at both the anode side and at the cathode side is above all necessary when starting up the operation of the fuel cells. In the operation of the fuel cells water is produced and takes care of the necessary humidification of the corresponding gases, as will later be explained in more detail in the description of embodiments. The humidifying devices are thus in principle only required when the fuel cells are taken into operation in order to positively influence the starting behavior of the fuel cells. The provision of the corresponding water containers is, however, problematic, because at temperatures below zero attention must be paid that the water contained in the water containers does not freeze. Proposals have already been made relating to the omission of the water containers and humidifying devices.
Even when humidifying devices are omitted, a problem exists, and indeed that the water present at the cathode side and at the anode side of the fuel cells, which forms in operation, condenses during cooling of the fuel cells and forms water droplets. A pronounced danger exists that at temperatures below zero the water droplets which separate out in this way freeze and, in operation or on recommencing the operation of the fuel cells, at least partly hinder the gas flow through the relevant, frequently narrowly dimensioned, passages and chambers.
Ice formation of this kind can hinder or prevent the throughflow of hydrogen and/or atmospheric oxygen, so that the operation or a renewed commencement of operation of the fuel cell system is prevented or only starts up very slowly. Moreover, the corresponding gases are not humidified, because the water has separated out in the form of ice. The lack of humidification of the gases can lead to the membranes in the PEM fuel cell being damaged.
It is the object of the present invention to provide a fuel cell system of the initially named kind and also a method for the operation of such a fuel cell system which makes it straightforwardly possible to permit operation at temperatures below zero, for example down to xe2x88x9240xc2x0 C., without the water which separates out hindering or preventing the required gas flow and without the membranes of the fuel cells being damaged or frost damage occurring in the fuel cell system. Furthermore, the necessity to provide a separate water container or a humidifying device should be avoided or, if they are present, to fully empty them at temperatures below zero.
Moreover, in accordance with the invention, the operation or the commencement of operation of the fuel cells should be possible at temperatures down to approximately xe2x88x9240xc2x0 C. without the start-up phase taking place very slowly.
A further object of the present invention is to always ensure (even at low temperatures), that the relative humidity in the fuel cells, i.e. in the gases which flow through the fuel cells, remains at an acceptable at least substantially constant value.
In order to satisfy this object provision is made in accordance with the invention that at least a part of the flow passages and/or flow chambers is provided with a coating which, at low temperatures, takes up water in distributed form and releases the water at least in part again at higher temperatures.
The invention is thus based on the recognition that there are coatings which, at low temperatures, take up water in distributed form, whereby the formation of water droplets and the coalescence of water droplets into larger quantities of water is prevented, with the distributed form of the water being so finely distributed that the water does not freeze even at temperatures of xe2x88x9240xc2x0 C. or less or, if it should freeze, does not lead to frost damage because of the small water volumes. Furthermore, it is recognized, in accordance with the invention, that the so stored quantities of water can be released again, at least partly, at higher operating temperatures and thus ensure the humidification of the corresponding gases.
Particularly favorable for a fuel cell is the fact that at low temperatures relatively little liberated water ensures the required relative humidity, whereas at higher temperatures a relatively large amount of water is released and can, even under these circumstances, ensure at least approximately the same relative humidity.
This signifies that the relative humidity in the gases or in the fuel cells can be kept at least substantially constant. This is ideal for the operation of a fuel cell. Excess water is given up with the exhaust gases of the fuel cells as water vapor. There is no need to store water in containers or to provide special humidifying devices in order to humidify the operating gases of the fuel cells. Simply and solely the provision of an appropriate coating thus makes it possible to dispense with humidifying devices and water containers at a stroke to enable the fuel cell system to operate at pronounced temperatures below zero such as, for example, xe2x88x9240xc2x0 C. and to ensure adequate humidification of the operating gases, which protects the membranes from damage and favors a more rapid start-up of the fuel cell system.
There are numerous different ways of realizing a corresponding coating.
It is for example known that silicates with the general formula
X12 [(Al 02)12(Si 02)12]xc2x7xH2O,
where X=Li, Na, K, Rb, or Cs and x is an integer, are in a position to take up relatively large quantities of water in the pores of the coating, with these pores having sizes in the nanometer range. The same applies to silicates of the general formula
Y12 [(Al 02)12(Si 02)12]xc2x7xH2O,
where Y=Be, Mg, Ca, Sr or Ba.
The coating can be realized as alkaline aluminum silicate or alkaline earth aluminum silicate, i.e. as so-called zeolites. The coating can moreover also comprise polysiloxanes, with a whole series of suitable of polysiloxanes being known per se in the field of gas chromatography.
Another possibility of realizing the coating is to provide this in the form of a polymer which is provided with acid radicals or alkaline radicals which have a chemical affinity for water. For example, the polymer can be a modified polyethylene glycol. Such coatings and similar coatings are used in so-called gas chromatographs and are deposited there as thin coatings on the inner wall of long capillaries. The technology thus exists of equipping both passages and chambers of smaller dimensions with corresponding coatings. Moreover, the named materials for the coatings have the advantage that they are obtainable at a favorable price.
Furthermore, the invention is concerned with a method of operating a fuel cell system with flow passages and/or flow chambers which conduct moist gases in operation, with the special characterizing feature that after the switching off of the fuel cell system at least a part of the water contained in the moist gases is absorbed by a coating which is located on the inner side of at least a part of the flow passages and/or spaces and is stored there, that at temperatures below 0xc2x0 C. freezing of the water does not occur or only occurs to a non-damaging extent and that, on recommencing the operation of the fuel cell system and the associated heating up, water which is stored by the coating is liberated again, at least in part, and is exploited for the required humidifying of the gases.
Particularly preferred embodiments of the fuel cell system or of the operating method can be found in the subordinate claims and also in the following description.